1. Field of the Invention
The present invention relates to earth-working equipment, such as an agricultural implement pulled by a tractor, and more particularly to a method for controlling a hydraulic system that operates a hitch that couples the agricultural implement to the tractor.
2. Description of the Related Art
A variety of agricultural implements are available to be pulled by a tractor for working earth in a farm field in which crops will be or have been planted. The implement is connected to a standard three-point hitch with right and left drag links on the rear of the tractor and the hitch can be operated to raise and lower the implement. The hitch is hydraulically driven by a system that typically provides one or both of position control and draft or load control.
The position control maintains the implement at a constant working depth in spite of differences in soil conditions. When employing solely position control, an operator input device is set to establish a desired vertical position of the three-point hitch with respect to a geometric plane established by the tractor. The operator input device sends a command signal to the hydraulic system to either raise or lower the hitch. A position feedback system senses the actual hitch position and stops the raising or lowering once the desired position is reached. This position is maintained by the control system until manually changed by the operator. A problem with maintaining a fixed position of the implement is that hard soil or an obstruction can exert such a large force that the tractor engine stalls.
Draft control raises and lowers the hitch during plowing so that the draft force that the implement exerts on the hitch remains constant, despite irregularities in the soil. Thus the draft control enables the working implement to operate effectively without stalling of the tractor engine even in the presence of obstacles such as stones. A tractor employing only draft control has a force sensor connected with the hydraulic system that elevates or lowers the implement hitch. This mechanism raises the hitch as the draft force increases and lowers the hitch as the draft force decreases. An operator input device establishes one or more parameters, such as the force threshold that must occur before the implement is raised of lowered.
In certain situations, each of these control techniques alone is not entirely satisfactory. Thus, some previous control systems employed both position and draft control. In that case, the position of the hitch is raised and lowered in response to changes in the sensed draft force, but the position is held within a range set by operator defined upper and lower threshold positions. The threshold range expands and contracts based on a draft setting provided by the operator. As the draft force increases, the hitch begins to rise until the upper threshold position is reached or until the draft force decreases. As the force decreases, the hitch lowers until the lower threshold position is reached or until the draft force increases.
The operator draft setting is influenced by a number of factors, including type of implement, commanded depth, soil composition, and soil moisture content. There often is no correlation between the operator setting and the specific position and draft force. These factors make the operator draft setting a trial and error proposition.
Another concern relates to adverse control effects that result from lateral forces acting on the implement. Some implements act to roll soil toward one side of the vehicle creating a load with a resultant force vector oriented at a significant angle away from the direction of vehicle travel. A hitch with right and left drag links observes this type of loading as positive load force on one drag link and negative load force on the opposite drag link. If these load forces are sensed and merely averaged, the control system does not recognize a change in loading and does not respond to changes in draft load, or operates at a significantly reduced level of sensitivity to these loading conditions.
As a consequence, there is a need for a hydraulic control system that provides an enhanced combination of position and draft control.